1. Field of the Invention
The present invention relates to an X-ray computed tomography apparatus and picture quality simulation apparatus.
2. Description of the Related Art
As is known, an X-ray computed tomography apparatus is designed to obtain an image (tomographic image) by calculating (reconstructing) the X-ray absorption coefficient of a tissue such as an organ on the basis of the amount of X-rays absorbed in a subject to be examined as an index called a CT value with reference to the X-ray absorption coefficient of water. A reconstructed image inevitably contains image noise. Image noise is typically defined with reference to a variation in the CT value of a homogeneous phantom image as a standard deviation, which is generally abbreviated as an image SD.
An image SD is determined in accordance with a plurality of condition items such as an imaging slice thickness, tube voltage, and tube current in scanning conditions, and a subject to be examined. In order to make diagnosis by observing a reconstructed image, e.g., to discriminate a small shadow on the image as noise or a tumor, the image SD of the image must be considered. In other words, scan condition items such as an imaging slice thickness, tube voltage, and tube current must be set to set the image SD to a proper value that allows discrimination between a tumor to be examined and noise.
As disclosed in Jpn. Pat. Appln. KOKAI Publication No. 11-235334, it is difficult to grasp the value of image SD only by inputting many condition items such as an imaging slice thickness, tube voltage, and tube current. In addition, even if an image SD can be grasped, many experiences and knowledge are required to recognize how much noise will occur with respect to the value of image SD. This is not an easy operation. It is therefore difficult to suitably set scan conditions.